The present invention relates to a hydrodynamic type porous oil-impregnated bearing and a bearing device, wherein a porous body is impregnated with lubricating oil or lubricating grease to have a self-lubricating function while floatably supporting a slide surface of a shaft by hydrodynamic oil film generated in a bearing clearance, the bearing and whose device are particularly suitable for use in a polygon mirror for laser beam printers, a spindle motor for magnetic disk drives, and the like which is required to rotate at high speed with a high degree of accuracy.
Porous oil-impregnated bearings are widely used as bearings having self-lubricability. However, since they are a kind of truly circular bearings, they tend to produce unstable vibrations when the shaft go eccentricity is small, inducing the so-called whirl in which the shaft is subjected to a revolving vibration at a rate which is half the rotary speed. As for measures against this, there may be cited as an example the formation of hydrodynazic-pressure-generating grooves, such as the herringbone or spiral type, in the bearing surface. Conventionally, hydrodynamic-pressure-generating grooves are in a porous oil-impregnated bearing to utilize the hydrodynamic pressure generated thereby to support the shaft while suppressing unstable vibrations, and examples of such arrangement are disclosed in Japanese Patent Publication No. Sho 64-11844 and Japanese Utility Model Publication No. Sho 63-19627.
Japanese Patent Publication No. Sho 64-11844 discloses a bearing device constructed by fitting a porous oil-impregnated member having herringbone grooves on a rotary shaft and combining it with a sleeve having cylindrical radial inner peripheral surface. On the other hand, Japanese Utility Model Publication No. Sho 63-19627 discloses the formation of glazed, hydrodynamic-pressure-generating grooves in the bearing surface of a porous oil-impregnated bearing.
According to Japanese Patent Publication No. Sho 64-11844, a porous oil-impregnated member having herringbone grooves is fitted on a rotary shaft with the intention of utilizing the centrifugal force attending the rotation of the shaft to cause exudation of oil into a bearing clearance. This construction, however, has the following drawbacks.
(1) The number of parts of the bearing device increases from the go usual two (rotary shaft and bearing) to three (rotary shaft, porous oil-impregnated member, and sleeve (bearing)), complicating the assembling operation and increasing the cost.
(2) In the case of a hydrodynamic bearing device, which requires a high degree of dimensional accuracy, there are as many as three parts, whose respective accuracies affect the post-assembly accuracy, so that it is more difficult to attain the intended accuracy than in the case of the two-part bearing device.
(3) During rotation, the centrifugal force continues acting on the porous oil-impregnated member. Therefore, the oil also continues exuding, so that the bearing clearance will be saturated sooner or later with oil, with continuing rotation forcing the oil to leak out of the bearing clearance. This means inevitable flow-away of oil.
According to Japanese Utility Model Publication No. Sho 63-19627, the bearing surface of a porous oil-impregnated bearing is formed with glazed, hydrodynamic-pressure-generating grooves. Such construction, however, has the following drawbacks.
(1) Since the grooves have their openings sealed, the circulation of oil, which is the greatest feature of the porous oil-impregnated bearing, is obstructed. Therefore, the oil which has been exuded in the bearing clearance is pressed into the bent regions of the grooves by the action of the herringbone grooves and stays there. A great shearing action is present in the bearing clearance, so that the oil staying in the oil grooves is subjected to this shearing force and friction heat, tending to be denatured, while a rise in temperature tends to cause premature oxidative deterioration of the oil. Therefore, the bearing life span is shortened. In contrast, in a usual porous oil-impregnated bearing, since the retained oil is circulated through the bearing clearance and bearing interior with the rotation of the shaft, it is not continuously subjected to the shearing force and since it is cooled even if once heated, there is little danger of the oil undergoing oxidative deterioration due to a rise in temperature.
(2) it is very difficult to apply a sealing treatment to the grooves. The published specification concerned states that sealing can be effected by plastic working. However, the depth of the hydrodynamic grooves is usually of the order of .mu.m; such compression molding is not effective in sealing the openings in the surface. Further, said published specification makes mention of coating as another means of plastic working; however, the thickness of the coating film has to be less than the groove depth, which means that it is very difficult to apply a coating film which is some .mu.m thick solely to the inclined region.